The present application relates to a nonvolatile memory using a magnetic material having a resistance change or the like, for example, as an information memory device and a recording method.
In an information device such as a computer, a DRAM (Dynamic Random Access Memory) that operates faster with higher density is widely used as a random access memory (RAM: Random Access Memory). However, since the DRAM is a volatile memory in which recorded information is lost when power supply is turned off, a nonvolatile memory in which information is left is desired.
Accordingly, as a candidate of the nonvolatile memory, a magnetoresistive random access memory (MRAM) that records information using magnetization of a magnetic material attracts attention.
In the MRAM, minute information memory devices that memorize information are regularly arranged and wired so that each of them may be accessible, and the wiring has a structure in which word lines and bit lines are provided, for example. Each information memory device has a memory layer in which information is memorized as a magnetization direction of ferromagnetic material.
Further, for the information memory device, a magnetic memory device that employs a structure using so-called magnetic tunnel junction (MTJ) is used. The magnetic tunnel junction includes the above described memory layer, a tunnel insulating film (nonmagnetic spacer film), and a magnetization fixed layer in which a magnetization direction is fixed. The magnetization direction in the magnetization fixed layer may be fixed by providing an antiferromagnetic layer, for example.
In the structure, so-called tunnel magnetic resistance effect that a resistance value for a tunnel current flowing in the tunnel insulating film changes depending on the angle formed by the magnetization direction of the memory layer and the magnetization direction of the magnetization fixed layer occurs. Using the tunnel magnetic resistance effect, readout of information can be performed. The magnitude of the resistance value takes the maximum value when the magnetization direction of the memory layer and the magnetization direction of the magnetization fixed layer are anti-parallel and takes the minimum value when they are in parallel.
A method of storing information in the information memory device (hereinafter, may be abbreviated as “writing of information” or “writing”) is performed in the following manner. That is, the magnetization direction of the memory layer of the information memory device is switched between a low-resistance state and a high-resistance state using a synthetic current magnetic field generated when currents are allowed to flow in both the word lines and the bit lines orthogonally arranged on and under the information memory device. Further, writing of information is performed using a difference of at least two resistance values. Generally, the difference between directions of magnetization (magnetization states) is stored in the information memory device in correspondence with “0” information and “1” information, respectively, at writing of information. In this manner, “0” information and “1” information are written by reversing the magnetization directions of the memory layer of including ferromagnetic matarial, high-speed and nearly infinite (>1015 times) rewriting can be performed.
On the other hand, readout of written information (hereinafter, may be abbreviated as “readout of information” or “readout”) is performed in the following manner. That is, selection of a memory cell is performed using an element such as a transistor and the difference between the magnetization directions of the memory layer is detected as a difference between voltage signals using the tunnel magnetic resistance effect of the information memory device. Thereby, the written information can be sensed.
However, in the MRAM, since address wiring for writing and address wiring for readout are necessary with respect to each information memory device, miniaturization of the memory cell is structurally difficult. Further, to rewrite the once written information, it is necessary to generate a relatively large current magnetic field and flow large currents to some degree (e.g., several milliamperes to several tens of milliamperes) in the address wirings. Thus, power consumption becomes greater. Furthermore, with miniaturization of the information memory device, also the address wirings become thinner, and it may be difficult to flow sufficient currents and the power consumption may increase because coercivity becomes larger and the necessary current magnetic field increases.
Accordingly, a memory having a configuration using magnetization reversal by spin transfer (also called “spin injection torque”) as a memory that enables magnetization reversal with less current, not depending on the current magnetic field, for storing information attracts attention. The magnetization reversal by spin transfer is to inject spin-polarized electrons while passing through a magnetic material into another magnetic material and cause magnetization reversal in the other magnetic material (e.g., see JP-A-2003-17782 and F. J. Albert et al., Applied Physics. Letters. Vol. 77, 2002, p. 3809).
This phenomenon is that, when the spin-polarized electrons that have passed through the magnetic layer with fixed magnetization direction (magnetization fixed layer) enters another magnetic layer without fixed magnetization direction (magnetization free layer), the electrons provide torque to the magnetization of the magnetic layer. Then, a current larger than a certain threshold level is flown in the other magnetic material, and thereby, the magnetization direction of the magnetic layer (magnetization free layer) can be reversed.
For example, a current is flown in a giant magneto resistive head (GMR element) or a magnetic tunnel junction element (MTJ element) having a magnetization fixed layer and a magnetization free layer in a direction perpendicular to its film surface. Thereby, the magnetization direction of at least a part of the magnetic layer of these elements can be reversed.
Further, an information memory device having a magnetization fixed layer and a magnetization free layer (memory layer) is formed and the polarity of a current flown in the information memory device is changed, and thereby, the magnetization direction of the memory layer is reversed and the low-resistance state and the high-resistant state are switched. Using the difference between the two resistance values, rewriting between “0” information and “1” information is performed.
On the other hand, readout of the written information can use the tunnel magnetic resistance effect like the MRAM by providing a tunnel insulating layer between a magnetization fixed layer and a magnetization free layer (memory layer).
The magnetization reversal by spin transfer has an advantage that, even when the information memory device is miniaturized, magnetization reversal can be realized without increase of current.
The absolute value of the current flown in the information memory device for magnetization reversal is 1 mA or less for the information memory device on the scale of about 0.1 μm, for example, decreases in proportion to the volume of the information memory device, and thus, that is advantageous in scaling. In addition, there is an advantage that word lines for memory, which are necessary in the MRAM, are not necessary and the configuration of the memory cell becomes simpler.
As below, an information memory device using spin transfer is called a spin MRAM (spin transfer random access memory). Further, a spin-polarized electron flow causing spin transfer is called a spin injection current. Great expectations are placed on the spin MRAM as a nonvolatile memory that enables lower power consumption and larger volume while maintaining the advantage of the MRAM that can realize high-speed and nearly infinite rewriting.
Further, a memory called an RRAM (resistance RAM) having a thin oxide between electrodes for changing the state of the oxide and performing recording by applying an electric field to the oxide and changing the electric resistance between the electrodes has been developed (e.g., see JP-A-2004-363604).
A nonvolatile memory in which information memory devices that can store information for long periods as electrical or structural changes are integrated mainly has two mechanisms. They are a mechanism of recording electric information in the information memory devices and a mechanism of reading out the accumulated information as a difference between at least two resistance values of electrically low-resistance state and high-resistance state. By applying a voltage, the difference between two potentials of a low-parallel magnetization state and an anti-parallel magnetization state is reduced and a magnetization change (reversal) occurs at a certain moment.